Abstract

Metastasis of cancer cells to distant sites leads to poor prognosis and comprises approximately 90% of cancer mortalities. It is evident that metastasis requires resistance to anoikis, an apoptotic program induced by detachment from the extracellular matrix. Heat shock proteins (HSPs) are molecular chaperones whose major functions include protein folding and suppression of apoptosis, and are transcriptionally regulated by heat shock transcription factor-1 (HSF1). HSF1 has been linked to metastasis and promotes anchorage-independent proliferation of breast cancer cells. HSP60 is a chaperone found in both the mitochondria and cytosol and is linked to metastasis and regulation of tumor cell apoptosis. To investigate whether HSF1 and HSP60 are necessary for anoikis resistance, anchorage-dependent lung carcinoma (A549, H460) and normal lung epithelial (Beas-2b) cells were suspended by plating on poly-2-hydroxyethyl-methacrylate-coated culture dishes. Anoikis induction was measured by executioner caspase-7 cleavage and PARP cleavage by western blot. Protein expression was measured by western blot. Small-interfering RNAs were used to inhibit hsf1 and hsp60. We show here that upon detachment for 24-48h, lung cancer cells resisted caspase-dependent anoikis in contrast to normal lung epithelial cells, with A549 cells showing the highest level of anoikis resistance. After 24h detachment there was increased phosphorylation of HSF1 on activating serine 326 in lung cancer cells. Antisense inhibition of hsf1 and hsp60 in attached lung cancer cells had minimal effects on caspase-7 and PARP cleavage. In contrast, hsf1 and hsp60 knockdown in cancer cells that were suspended for 24-48h initiated caspase-dependent anoikis. Furthermore, knockdown of hsf1 resulted in inhibition of HSP72, HSP40, and HSP27. HSP60 was not significantly inhibited with loss of HSF1. This data suggests that HSF1 and HSP60 are necessary for lung carcinoma anoikis resistance. Our work adds to evidence suggesting HSF1 and HSP60 are promising therapeutic targets for metastasis prevention. Although more detailed mechanistic evaluation is necessary, a therapeutic approach to induce anoikis in circulating tumor cells by modulating HSF1 or HSP60 may be promising.